After seeing the sony recorder in your picture I bought the 50 buck version to do the same thing.
Unfortunately the better Sony recorder has microscopic forward and back switches with no exposed pc board trace points to solder anything to.
So $50 down the drain. Will try another cheap mp3 player to hopefully find something to solder a remote advance switch to.

Not a happy camper.
Really want a working virtual caller to help my practice.
Arrrgggg.